1. Field of the Invention
This invention relates to an improvement on a camera of the kind having automatic focus detecting device arranged to form a measured distance data required for adjusting a photo-taking optical system into an in-focus state and a focusing data indicative of a de-focused state, etc.
2. Description of the Prior Art
Cameras of the kind having an automatic focus detecting device which forms a focusing data for adjustment of a photo-taking optical system to an in-focus state are generally arranged to obviate the necessity of performing a manual focusing operation and to accomplish a photo-taking operation when a shutter button is just pushed with the image of an object to be photographed set within a distance measurement mark provided at a middle part of a view finder.
However, it has been a shortcoming of the camera of this kind that an in-focus picture of a desired object cannot be taken unless the object is within the distance measurement mark of the view finder when the shutter button is pushed. This shortcoming has deprived the photographers of having a wide picture composing latitude when they wish, for example, to have a main object placed at a corner of a picture. The same shortcoming has presented another problem that, in the event of a continuous photographing operation, the object might move out of the distance measurement mark of the view finder and come out of focus.
To eliminate this shortcoming of the camera having an automatic focus detecting device, there has been proposed a camera having a so-called pre-focus device which is characterized by the arrangement that detection of a focusing data and retention thereof can be accomplished by just pushing the shutter button halfway downward. In this case, the camera is operated as follows: The object as viewed from a view finder is placed inside the distance measurement mark and the shutter button is pushed halfway to obtain a focusing data. Then, while leaving the shutter button in the half pushed state, a picture is composed as desired. After a picture composition is thus determined, the shutter button is further pushed. In this manner, the object can be photographed always in a sharply focused state wherever the object is located within the view finder. In this case, however, for retaining the same focusing data, the shutter button is left in the half pushed state which is very unstable. Therefore, while the camera is shifted in composing a picture, the shutter button might be released from the halfway operation and thus might unintentionally be brought back to its original position. Besides, the switch of a circuit for holding the focusing data might come to chatter if the force of a finger pushing it is momentarily lost under a vibrant condition, etc. in further pushing the shutter button for photographing. Then, the chattering of the switch lets the retained data disapper before use. Further, while the shutter button is thus kept in the half pushed state, the photographer might by mistake let the shutter button come back to its original position before pushing it to a full stroke. These misoperations are sometimes unconsciously performed to find a blurred photograph later on. Even if the photographer comes to note such a mis-operation before photographing, he or she would miss a valuable shutter chance while attempting to obtain a focusing data once again.
In another known example of improvement which has been disclosed, for example, in Japanese Laid-Open Patent application No. SHO 56-1024, the focusing data is arranged to be retained by means of a switch arranged to be operated independently of the shutter button. This improvement, however, has presented the following problems: An oversight or negligence to turn off the above-stated switch would waste electric energy. This problem is a very serious for cameras which are allowed to use batteries of a limited capacity. Further, if a next photographing operation is performed with the switch having been not turned off by an oversight after a photographing operation, the new photgraphing operation would be performed on the basis of the focusing data previously obtained.